Metal structures such as ships, bridges, construction machines, industrial machines, offshore structures, and automobiles are made by welding together many metal members. At these welded portions, welded joints are formed using various welding methods.
However, in such a welded joint, at the boundary part where the surface of the weld metal forming the weld bead intersects a surface of a metal member (base material) (referred to as the toe of the weld bead) and its vicinity (hereinafter referred to as the toe portion of the weld bead), tensile residual stress easily remains due to cooling in the state where the high temperature state weld metal is restrained by the surrounding base material. Furthermore, when used as a structure, this becomes a part where stress easily concentrates due to external force applied to the member.
Therefore, a welded joint used in a metal structure may suffer from fatigue cracks occurring from the toe portion of the weld bead and developing into critical cracks and fractures due to repeated load. Further, residual stress and stress concentration at the toe portion of the weld bead impedes improvement of fatigue characteristics of a metal structure.
Accordingly, fatigue cracks occurring in such a welded joint have a serious effect on the reliability of the entire structure, so a variety of methods for improving fatigue characteristics of welded joints have been attempted in the past. (For example, see Non-Patent Literatures 1 and 2.)
Specifically, the following Non-Patent Literatures 1 and 2 propose methods of reducing stress concentration at weld zones by (a) the method of using a mechanical method (grinding) to smooth the weld zone and (b) the method of using TIG welding to dress the weld zone.
Further, there is also proposed a method of treating the weld zone by peening (impact) to introduce compressive stress to portions where fatigue cracks occur and reduce stress concentration. As a specific impact treatment, (c) shot peening, (d) hammer peening, and also, in recent years, (e) ultrasonic impact treatment (for example, see Patent Literatures 1 to 3) may be mentioned.
Further, a method treating the vicinity of the weld toe portion by peening (impact) to improve the metal structure of the weld heat affected zone near the fusion line and improve the toughness of the heat affected zone is disclosed in Patent Literature 4. However, this is for improving the material quality at the starting point of brittle fracture based on brittle fractures generally forming from defects remaining on the fusion line of the weld zone and does not improve the fatigue characteristics.
Further, as methods for improving the fatigue characteristics of a welding toe portion at an end of a rib plate attached by welding, methods using a compression punch or the like to apply compressive residual stress to the welding toe portion (Patent Literatures 5 and 6) are disclosed, however, these methods both are methods for improving the fatigue characteristics at the end of a rib plate subjected to boxing etc. and cannot be applied to the part mainly covered by the present invention, that is, the welding toe portion which continues long in the weld line direction.